The present invention relates to a new silicon thin film, a group of silicon single crystal grains, and formation processes thereof, and a semiconductor device, a flash memory cell, and fabrication processes thereof.
A silicon thin film composed of a group of silicon single crystal grains formed on a base body has been used for various kinds of semiconductor devices such as a thin film transistor (hereinafter, referred to as a xe2x80x9cTFTxe2x80x9d) and a semiconductor device based on a SOI (Silicon On Insulator) technique, and for solar cells; and it is being examined to be applied to production of micromachines.
In the field of semiconductor devices, for example, a stacked SRAM using TFTs as load elements has been proposed. TFTs have been also used for LCD (Liquid Crystal Display) panels. And, in general, a silicon thin film composed of a group of silicon single crystal grains is used for a TFT required to be enhanced in electric characteristics such as a carrier mobility (xcexc), conductivity ("sgr"), on-current characteristic, subthreshold characteristic, and on/off current ratio. Concretely, efforts are being made to improve characteristics of SRAMs and TFTs by increasing sizes of silicon single crystal grains in combination with reduction in density of twin crystals for lowering a trap density in the silicon single crystal grains.
The enlargement in size (up to 1 xcexcm) of silicon single crystal grains for improving the electric characteristics of such a silicon thin film has been examined by a SPC (Solid Phase Crystallization, solid phase crystallization from amorphous silicon) technique or an ELA (Excimer Laser Anneal, liquid phase crystallization using excimer laser). One process of forming a silicon thin film using the ELA technique has been known, for example, from a document [xe2x80x9cDependence of Crystallization Behaviors of Excimer Laser Annealed Amorphous Silicon Film on the Number of Laser Shotxe2x80x9d, B. Jung, et al., AM-LCD 95, PP. 177-120]. This document describes that a silicon thin film composed of silicon single crystal grains whose selected orientation is approximately the  less than 111 greater than  direction can be formed by repeatedly irradiating excimer laser beams on an amorphous silicon layer. Another process of forming a silicon thin film using the ELA technique has been also known, for example, from a document [xe2x80x9cCrystal forms by solid-state recrystallization of amorphous Si film on SiO2xe2x80x9d, T. Noma, Appl. Phys. Lett. 59 (6), Aug. 5, 1991,pp. 653-655]. This document describes that silicon single crystal grains are oriented in the  less than 110 greater than  direction, and they include fine {111} twin crystals.
A process of forming a silicon thin film by graphoepitaxial growth using a strip-heater has been known, for example, from a document [Silicon graphoepitaxy using a strip-heater ovenxe2x80x9d, M. W. Geis, et al., Appl. Phys. Lett. 37(5), Sep. 1, 1980, pp. 454-456]. This document describes that a silicon thin film formed on SiO2 is composed of a (100) aggregation structure.
A silicon thin film composed of a group of silicon single crystal grains has been also formed by a chemical-vapor deposition (CVD) process or a random solid-phase growth process. For example, the formation of polysilicon crystal grains by CVD has been known from Japanese Patent Laid-open Nos. Sho 63-307431 and Sho 63-307776. In the techniques disclosed in these documents, the selected orientation of silicon single crystal grains is the  less than 111 greater than  direction. Incidentally, in the case where a silicon thin film composed of a group of silicon single crystal grains having large sizes is formed by a normal chemical vapor deposition process, it cannot satisfy a uniform quality, a reduced leak, and a high mobility. In the random solid-phase growth process, it is possible to form a silicon thin film composed of a group of silicon single crystal grains having an average grain size of 1 xcexcm or more; however, it is difficult for silicon single crystal grains to selectively grow. Further, in the TFT using the silicon thin film formed by such a process, since grain boundaries tend to be present in a TFT active region, there occurs a problem that TFT characteristics are varied depending on the presence of the grain boundaries, to thereby shorten the life time of the TFT.
In all of the techniques disclosed in the above-described references, no attempts has been not made to regularly arrange a group of silicon single crystal grains on an insulating film. If a group of silicon single crystal grains can be regularly arranged on an insulating film, the TFT characteristics can be highly controlled and equalized, and also one TFT can be formed in each of the silicon single crystal grains. This is expected to further develop the SOI technique.
A process of arranging silicon nuclei or crystal nuclei at desired positions and forming silicon single crystal grains having large sizes on the basis of the silicon nuclei or crystal nuclei has been known, for example, from Japanese Patent Laid-open Nos. Hei 3-125422, Hei 5-226246, Hei 6-97074, and Hei 6-302512. In the technique disclosed in Japanese Patent Laid-open No. Hei 3-125422, micro-sized silicon nuclei or crystal nuclei must be formed by patterning using a lithography process; however, there is a limitation to accurately form these micro-sized silicon nuclei or crystal nuclei by the present lithography technique. In the case where the sizes of silicon nuclei or crystal nuclei are large, polycrystals tend to be formed with twin crystals and dislocations easily produced, resulting in the reduced throughput. In the techniques disclosed in Japanese Patent Lid-open Nos. Hei 5-226246, Hei 6-97074, and Hei 6-302512, it is necessary to irradiate an energy beam enabling fine convergence and direct scanning onto an amorphous silicon layer or to carry out ion implantation. Accordingly, these techniques have problems that not only the step of forming silicon single crystal grains is complicated, but also it takes a lot of time to form silicon single crystal grains because of the necessity of a solid phase growth step, resulting in the reduced throughput.
On the other hand, non-volatile memories are being extensively developed at present. In particular, a flash memory having a floating gate structure is being examined from the viewpoint of the reduced size of the memory cell and the lowered voltage. In a flash memory, data is written or erased by injecting or discharging an electric charge into or from the floating gate. Of various electric charge injecting methods, a channel hot electron injection method or a method of allowing a Fowler-Nordheim""s tunnel current to flow by applying a high electric field (for example, 8 MV/cm or more) on a tunnel oxide film are generally used.
In such a flash memory cell, it has been known that a threshold voltage after erasion of data is varied depending on variations in sizes of polycrystalline silicon grains forming a floating gate, for example, from a document [xe2x80x9cNon-volatile Memory and Its Scalingxe2x80x9d, Journal of Japan Society of Electron Information Communication, Vol. 9, No. 5, pp. 469-484 (May, 1996)]. Further, as one means for realizing a future fine flash memory cell operated at a low voltage, a flash memory including a floating gate composed of silicon nanocrystals has been proposed in a document [xe2x80x9cA silicon nanocrystal based memoryxe2x80x9d, S. Tiwari, et al., Appl. Phys. Lett. 68 (10), 4, pp. 1377-1379, Mar. 4, 1996]. Additionally, as one form of a non-volatile memory to lead the next generation over the present semiconductor device, a single electron memory operated at a low voltage using a small storage electric charge (electron) has been proposed in a document [xe2x80x9cA Room-temperature Single-Electron Memory Device Using Fine-Grain Polycrystalline Siliconxe2x80x9d, K. Yano, et. al., IEDM93, PP. 541-544].
To realize a flash memory cell hard to be varied in a threshold voltage after erasion of data, it is necessary to make as small as possible variations in sizes of silicon crystal grains forming a floating gate. Also, to realize a fine flash memory cell operated at a low voltage, it is necessary to regularly form fine silicon crystal grains on a thin insulating film (tunnel oxide film) with an excellent controllability.
An object of the present invention is to provide a process of easily, effectively forming a group of silicon single crystal grains in a grid pattern on a base body with the reduced variations in grain size, and a group of silicon single crystal grains formed by the process.
Another object of the present invention is to provide a process of easily, effectively forming a silicon thin film composed of a group of silicon single crystal grains which are arranged in a grid pattern on a base body with the reduced variations in grain size, and a silicon thin film formed by the process.
A further object of the present invention is to provide a process of fabricating a semiconductor device using the above silicon thin film or the above group of silicon single crystal grains, and a semiconductor device fabricated by the process.
An additional object of the present invention is to provide a process of fabricating a flash memory cell using the above silicon thin film or the above group of silicon single crystal grains, and a flash memory cell fabricated by the process.
To achieve the above object, according to the present invention, there is provided a process of forming a silicon thin film, including the step of: irradiating a pulsed rectangular ultraviolet beam on an amorphous or polycrystalline silicon layer formed on a base body, to thereby form a silicon thin film composed of a group of silicon single crystal grains on the base body; wherein the moved amount or amount of movement (L) of a ultraviolet beam irradiating position in a period from completion of an irradiation of the rectangular ultraviolet beam to starting of the next irradiation of the rectangular ultraviolet beam is specified at 40 xcexcm or less, and a ratio (R=L/W) of the moved amount to a width (W) of the rectangular ultraviolet beam measured in the movement direction thereof is in a range of 0.1 to 5%, preferably, in a range of 0.5 to 2.5%, whereby forming a silicon thin film composed of a group of silicon single crystal grains which are each approximately rectangular-shaped and which are arranged in a grid pattern on the base body, a selected orientation of the silicon single crystal grains to the surface of the base body being approximately the  less than 100 greater than  direction.
According to the present invention, there is provided a silicon thin film including a group of silicon single crystal grains which are each approximately rectangular-shaped and which are arranged in a grid pattern on a base body, wherein a selected orientation of the silicon single crystal grains to the surface of the base body is approximately the  less than 100 greater than  direction.
To achieve the above object, according to the present invention, there is provided a process of fabricating a semiconductor device, including the steps of: irradiating a pulsed rectangular ultraviolet beam on an amorphous or polycrystalline silicon layer formed on a base body, to form a silicon thin film composed of a group of silicon single crystal grains on the base body; and forming a source/drain region and a channel region in the silicon thin film or the silicon single crystal grains;wherein the moved amount (L) of a ultraviolet beam irradiating position in a period from completion of an irradiation of the rectangular ultraviolet beam to starting of the next irradiation of the rectangular ultraviolet beam is specified at 40 xcexcm or less, and a ratio (R=L/W) of the moved amount to a width (W) of the rectangular ultraviolet beam measured in the movement direction thereof is in a range of 0.1 to 5%, preferably, in a range of 0.5 to 2.5%, whereby forming a silicon thin film composed of a group of silicon single crystal grains which are each approximately rectangular-shaped and which are arranged in a grid pattern on the base body, a selected orientation of the silicon single crystal grains to the surface of the base body being approximately the  less than 100 greater than  direction.
According to the present invention, there is provided a semiconductor device including a source/drain region and a channel region formed in a silicon thin film composed of a group of silicon single crystal grains which are each approximately rectangular-shaped and which are arranged in a grid pattern on a base body or formed in the silicon single crystal grains, wherein a selected orientation of the silicon single crystal grains to the surface of the base body is approximately the  less than 100 greater than  direction.
As the semiconductor device of the present invention or the semiconductor device fabricated by the process of fabricating a semiconductor device of the present invention, there may be exemplified a top gate type or a bottom gate type thin film transistor used for LCD panels, a semiconductor device based on the SOI technique (for example, a thin film transistor as a load element of a stacked SRAM), and a MOS type semiconductor device. The silicon thin film and the formation process thereof according to the present invention can be applied not only to fabrication of these semiconductor devices but also to production of solar cells and to fabrication of micromachines.
In the silicon thin film and the formation process thereof, and the semiconductor device and the fabrication process thereof according to the present invention, a length of one side of a silicon single crystal grain approximately rectangular-shaped may be 0.05 xcexcm or more, preferably, 0.1 xcexcm or more. Here, the wording xe2x80x9ca silicon single crystal grain approximately rectangular-shapedxe2x80x9d means not only a perfectly rectangular silicon single crystal grain but also an imperfectly rectangular silicon single crystal grain having a chipped corner. The length of one side of an imperfectly rectangular silicon single crystal grains having a chipped corner means the length of one side of a virtual rectangular single crystal grain obtained by filling up the chipped corner. The same shall apply hereinafter. The average thickness of a silicon thin film may be in a range of 1xc3x9710xe2x88x928 m to 1xc3x9710xe2x88x927 m, preferably, in a range of 1xc3x9710xe2x88x928 m to 6xc3x9710xe2x88x928 m, more preferably, in a range of 1xc3x9710xe2x88x928 m to 4xc3x9710xe2x88x928 m. When the average thickness of a silicon thin film is less than 1xc3x9710xe2x88x928 m, there is a difficulty in fabrication of a semiconductor device using the silicon thin film. On the other hand, when it is more than 1xc3x9710xe2x88x927 m, the thickness of an amorphous or polycrystalline silicon layer required to ensure the thickness of the silicon thin film is excessively thick, and consequently, the selected orientation of the silicon single crystal grains is possibly out of approximately the  less than 100 greater than  direction. The average thickness of a silicon thin film may be measured by an ellipsometer, interference spectrometer, or the like.
In the silicon thin film and the semiconductor device using the silicon thin film according to the present invention, a group of silicon single crystal grains constituting the silicon thin film may be formed by irradiating a pulsed rectangular ultraviolet beam on an amorphous or polycrystalline silicon layer formed on a base body, and the moved amount (L) of a ultraviolet beam irradiating position in a period from completion of an irradiation of the rectangular ultraviolet beam to starting of the next irradiation of the rectangular ultraviolet beam may be specified at 40 xcexcm or less, and a ratio (R=L/W) of the moved amount to a width (W) of the rectangular ultraviolet beam measured in the movement direction thereof may be in a range of 0.1 to 5%, preferably, in a range of 0.5 to 2.5%. In the silicon thin film and the formation process thereof, and the semiconductor device and the fabrication process thereof according to the present invention, opposed sides of the silicon single crystal grain approximately rectangular-shaped may be approximately in parallel to the movement direction of the ultraviolet beam irradiating position or intersect the movement direction of the ultraviolet beam irradiating position at approximately 45xc2x0. Crystal planes constituting these two sides are the {220} planes. That is, a crystal plane constituting one side of a silicon single crystal grain approximately rectangular-shaped is the {220} plane.
To achieve the above object, according to the present invention, there is provided a process of forming a group of silicon single crystal grains including: a step (a) of irradiating a pulsed rectangular ultraviolet beam on an amorphous or polycrystalline silicon layer formed on a base body, to thereby form a silicon thin film composed of a group of silicon single crystal grains which are each approximately rectangular-shaped and which are arranged in a grid pattern on the base body, a selected orientation of the silicon single crystal grains to the surface of the base body being approximately the  less than 100 greater than  direction; and a step (b) of separating adjacent ones of the silicon single crystal grains to each other; wherein the moved amount (L) of a ultraviolet beam irradiating position in a period from completion of an irradiation of the rectangular ultraviolet beam to starting of the next irradiation of the rectangular ultraviolet beam is specified at 40 xcexcm or less, and a ratio (R=L/W) of the moved amount to a width (W) of the rectangular ultraviolet beam measured in the movement direction thereof is in a range of 0.1 to 5%, preferably, 0.5 to 2.5%.
According to the present invention, there is provided a group of silicon single crystal grains, including a plurality of silicon single crystal grains which are each approximately rectangular-shaped and which are arranged in a grid pattern on a base body, wherein a selected orientation of the silicon single crystal grains to the surface of the base body is approximately the  less than 100 greater than  direction, and adjacent ones of the silicon single crystal grains are separated from each other.
To achieve the above object, according to the present invention, there is provided a process of fabricating a flash memory cell, including: a step (a) of irradiating a pulsed rectangular ultraviolet beam on an amorphous or polycrystalline silicon layer formed on a tunnel oxide film, to form a silicon thin film composed of a group of silicon single crystal grains which are each approximately rectangular-shaped and which are arranged in a grid pattern on the tunnel oxide film, a selected orientation of the silicon single crystal grains to the surface of the tunnel oxide film is approximately the  less than 100 greater than  direction; and a step (b) of separating adjacent ones of the silicon single crystal grains to each other, whereby forming a floating gate composed of the group of silicon single crystal grains; wherein the moved amount (L) of a ultraviolet beam irradiating position in a period from completion of an irradiation of the rectangular ultraviolet beam to starting of the next irradiation of the rectangular ultraviolet beam is specified at 40 xcexcm or less, and a ratio (R=L/W) of the moved amount to a width (W) of the rectangular ultraviolet beam measured in the movement direction thereof is in a range of 0.1 to 5%, preferably, 0.5 to 2.5%.
According to the present invention, there is provided a flash memory cell including a floating gate composed of a plurality of silicon single crystal grains which are each approximately rectangular-shaped and which are formed on a tunnel oxide film, a selected orientation of the silicon single crystal grains to the surface of the tunnel oxide film being approximately the  less than 100 greater than  direction; wherein the silicon single crystal grains are arranged in a grid pattern on the tunnel oxide film and adjacent ones of the silicon single crystal grains are separated from each other. In addition, the thickness of each of the silicon single crystal grains separated from each other may be in a range of 1xc3x9710xe2x88x928 m to 11xc3x9710xe2x88x927 m, preferably, 1xc3x9710xe2x88x928 m to 8xc3x9710xe2x88x928 m, more preferably, in a range of 2xc3x9710xe2x88x928 m to 5xc3x9710xe2x88x928 m.
The flash memory cell of the present invention or the flash memory cell fabricated by the process of fabricating a flash memory cell of the present invention basically includes a source/drain region and a channel region formed in a semiconducting substrate or a silicon layer, a tunnel oxide film formed thereon, a floating gate formed on the tunnel oxide film, an insulating film covering the floating gate, and a control gate.
In the group of silicon single crystal grains or the flash memory cell using the same according to the present invention, the group of silicon single crystal grains may be formed by a process including: a step (a) of irradiating a pulsed rectangular ultraviolet beam on an amorphous or polycrystalline silicon layer formed on the base body (or tunnel oxide film), to thereby form a silicon thin film composed of a group of silicon single crystal grains which are each approximately rectangular-shaped and which are arranged in a grid pattern on the base body (tunnel oxide film), a selected orientation of the silicon single crystal grains to the surface of the base body (tunnel oxide film) being approximately the  less than 100 greater than  direction; and a step (b) of separating adjacent ones of the silicon single crystal grains to each other; wherein the moved amount (L) of a ultraviolet beam irradiating position in a period from completion of an irradiation of the rectangular ultraviolet beam to starting of the next irradiation of the rectangular ultraviolet beam is specified at 40 xcexcm or less, and a ratio (R=L/W) of the moved amount to a width (W) of the rectangular ultraviolet beam measured in the movement direction thereof is in a range of 0.1 to 5%, preferably, in a range of 0.5 to 2.5%.
In the group of silicon single crystal grains and the formation process thereof, and the flash memory cell and the fabrication process thereof according to the present invention, the step (b) of separating adjacent ones of the silicon single crystal grains to each other preferably includes a step of oxidizing the silicon thin film formed in the step (a) to form each region made of silicon oxide between the adjacent ones of the silicon single crystal grains. Alternatively, the step (b) of separating adjacent ones of the silicon single crystal grains to each other preferably includes a step of etching the silicon thin film formed in the step (a) to form each space between the adjacent ones of the silicon single crystal grains. The length of one side of each of the approximately rectangular-shaped silicon single crystal grains in the silicon thin film formed in the step (a) is preferably as short as possibly; however, it is preferably 0.05 xcexcm or more from a practical standpoint. The average thickness of the silicon thin film formed in the step (a) may be in a range of 1xc3x9710xe2x88x928 m to 1xc3x9710xe2x88x927 m, preferably, 1xc3x9710xe2x88x928 m to 6xc3x9710xe2x88x928 m, more preferably, 1xc3x9710xe2x88x928 m to 4xc3x9710xe2x88x928 m. The opposed sides of each of the approximately rectangular-shaped silicon single crystal grains in the silicon thin film formed in the step (a) may be approximately in parallel to the movement direction of the ultraviolet beam irradiating position or intersect the movement direction of the ultraviolet beam irradiating position at approximately 45xc2x0. Crystal planes constituting these two sides are the {220} planes. That is, a crystal plane constituting one side of a silicon single crystal grain approximately rectangular-shaped is the {220} plane.
As the base body or the tunnel oxide film in the present invention, there may be exemplified, while not exclusively, silicon oxide (SiO2), silicon nitride (SiN), SiON, a stacked structure of SiO2xe2x80x94SiN, and a stacked structure of SiO2xe2x80x94SiNxe2x80x94SiO2. The base body or the tunnel oxide film may be formed by oxidization or nitrization of the surface of a silicon semiconducting substrate, or may be formed of a suitable film on a semiconducting substrate, a layer, or an interconnection layer by CVD or the like.
As a ultraviolet beam, there may be exemplified a XeCl excimer laser having a wavelength of 308 nm and a full-solid ultraviolet laser. The width (W) of a rectangular ultraviolet beam measured in the movement direction is preferably in a range of 40 xcexcm to about 1 mm. The length of a rectangular ultraviolet beam measured in the direction perpendicular to the movement direction may be freely selected. It is desired to use a ultraviolet beam having an extremely sharp rise in energy at an edge portion. As a ultraviolet beam source for generating such a ultraviolet beam, there may be exemplified, while not exclusively, a combination of a XeCl excimer laser source, and attenuator, a beam homogenizer for equalizing a rectangular beam, and a reflection mirror.
When the moved amount (L) of a ultraviolet beam irradiating position in a period from completion of an irradiation of a rectangular ultraviolet beam to stating of the next irradiation of the rectangular ultraviolet beam is more than 40 xcexcm or a ratio (R=L/W) of the moved amount to the width (W) of the ultraviolet beam measured in the movement direction of the ultraviolet beam irradiating position is more than 5%, there is a fear that a group of silicon single crystal grains which are each approximately rectangular shaped and which are arranged in a grid pattern on a base body are not formed, or that the selected orientation of the silicon single crystal grains to the surface of the base body is out of approximately the  less than 100 greater than  direction. Further, when the movement ratio (R=L/W) is less than 0.1% of the width (W) of the ultraviolet beam measured in the movement direction of the ultraviolet beam irradiating position, the throughput becomes excessively low. In addition, the base body may be moved with the ultraviolet beam source kept fixed; the ultraviolet beam source may be moved with the base body kept fixed; or both the ultraviolet beam source and the base body may be moved.
In the case where 30% or more of silicon single crystal grains constituting a group of silicon single crystal grains are selectively oriented approximately in the  less than 100 greater than  direction with respect to the surface of a base body, the selected orientation of the silicon single crystal grains constituting the group of silicon single crystal grains to the surface of a base body is specified as approximately the  less than 100 greater than  direction. In addition, the approximately  less than 100 greater than  direction of silicon single crystal grains contains the case that the  less than 100 greater than  direction of the silicon single crystal grains is not strictly in parallel to the direction perpendicular to the surface of a base body. The selected orientation is sometimes called xe2x80x9ca preferred orientationxe2x80x9d. A polycrystalline structure in the form of a film or the like in which crystals are not random-oriented but a large number of the crystals have a crystal axis, crystal plane and the like oriented in a specified direction, is called xe2x80x9can aggregate structurexe2x80x9d or xe2x80x9ca fiber structurexe2x80x9d. In this structure, the oriented crystal axis is called the selected orientation.
In the present invention, as described above, by specifying the moved amount (L) of a ultraviolet beam irradiating position in a period from completion of an irradiation of a rectangular ultraviolet beam to starting of the next irradiation of the rectangular ultraviolet beam to be 40 xcexcm or less, and also specifying a ratio (R=L/W) of the moved amount to the width (W) of the ultraviolet beam measured in the movement direction of the ultraviolet beam irradiating position to be in a range of 0.1 to 5%, there can be formed a silicon thin film composed of a group of silicon single crystal grains which are each approximately rectangular-shaped and which are arranged in a grid pattern on a base body, a selected orientation of the silicon single crystal grains being approximately the  less than 100 greater than  direction. The reason for this is unclear, but it may be considered as follows: namely, by irradiating a pulsed rectangular ultraviolet beam (having extremely sharp rise in energy at an edge portion) in a certain region of an amorphous or polycrystalline silicon layer while overlapping and slightly shifting the ultraviolet beams, there is established a repetition of a thermal equilibrium state by heat reservation and a cooling (solidifying) state, to thereby form a group of these silicon single crystal grains. Also, the reason why the selected orientation of silicon single crystal grains to the surface of a base body is approximately the  less than 100 greater than  direction may be considered to be due to the free energy on the surface of Si to the base body made of, for example, SiO2.
According to the present invention, there can be easily, effectively formed a silicon thin film composed of a group of silicon single crystal grains arranged in a grid pattern on a base body (insulating film), a selected orientation of the silicon single crystal grains to the surface of the base body being approximately the  less than 100 greater than  direction. Accordingly, it becomes possible to highly control and equalize characteristics of a TFT formed of the silicon thin film or to improve a semiconductor device based on the SOI technique by forming a TFT in a fine silicon single crystal grain. Further, since the crystallinity of the silicon thin film is improved in terms of macro-structure, characteristics of a TFT used for LCD panels and the like can be enhanced. Also, there can be realized a flash memory cell (nano dot memory) capable of being operated at a low voltage by directly applying a tunneling effect and electron accumulation. Additionally, by forming a floating gate of a flash memory cell of a silicon thin film of the present invention, variations in sizes of silicon grains forming the floating gate can be reduced, so that there can be realized a flash memory cell hard to be varied in threshold voltage after erasion of data.